Dissociation Dissociation is separation of ions that occurs when an ionic compound dissolves. Chapter 13 Section 1 Compounds in Aqueous Solution 1 mol

Dissociation

• Dissociation is separation of ions that occurs when an ionic compound dissolves.

+⏐ ⏐ ⏐→s aq + aq2H O –NaCl( ) Na ( ) Cl ( )

+⏐ ⏐ ⏐→s aq + aq2H O 2 –2CaCl ( ) Ca ( ) 2Cl ( )

Chapter 13Section 1 Compounds in Aqueous Solution

1 mol 1 mol 1 mol

1 mol 1 mol 2 mol

Dissociation of NaCl


Dissociation, continued

Sample Problem A

Write the equation for the dissolution of aluminum sulfate, Al2(SO4)3 , in water. How many moles of aluminum ions and sulfate ions are produced by dissolving 1 mol of aluminum sulfate? What is the total number of moles of ions produced by dissolving 1 mol of aluminum sulfate?


Dissociation, continued

Sample Problem A SolutionGiven: amount of solute = 1 mol Al2(SO4)3

solvent identity = water

Unknown: a. moles of aluminum ions and sulfate ions

b. total number of moles of solute ions produced

Solution:

s aq + aq2H O 3 2–2 4 3 4Al (SO ) ( ) 2Al ( ) 3SO ( )+⏐ ⏐ ⏐→

+ 3 2–2 4 3 41 mol Al (SO ) 2 mol Al 3 mol. SOa +→

+ 3 2–42 mol Al 3 mol SO 5 mol of solute i. sb on+ =


Dissociation, continuedPrecipitation Reactions

• Although no ionic compound is completely insoluble, compounds of very low solubility can be considered insoluble for most practical purposes.


General Solubility Guidelines


Click below to watch the Visual Concept.

Visual Concept

Chapter 13

Rules for Solubility

Section 1 Compounds in Aqueous Solution

Soluble and Insoluble Ionic Compounds


Particle Model for the Formation of a Precipitate



Visual Concept

Chapter 13

Precipitation Reactions


Dissociation, continuedNet Ionic Equations

• A net ionic equation includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution.

• Ions that do not take part in a chemical reaction and are found in solution both before and after the reaction are spectator ions.


Dissociation, continuedNet Ionic Equations, continued

Overall ionic equation

aq + aq + aq + aq

s + aq + aq

2 3– 4 2–

3– 4

Cd ( ) 2NO ( ) 2NH ( ) S ( )

CdS( ) 2NO ( ) 2NH ( )

+ +

+

→

aq + aq s 2 2–Cd ( ) S ( ) CdS( )+ →


net ionic equation

Writing a Net Ionic Equation



Chapter 13

Net Ionic Equation



Sample Problem B

Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined. Write the equation for the possible double-displacement reaction. Then write the formula equation, overall ionic equation, and net ionic equation for the reaction.


Sample Problem B SolutionGiven: identity of reactants: zinc nitrate and ammonium sulfide

reaction medium: aqueous solution

Unknown: a. equation for the possible double-displacement

reaction

b. identity of the precipitate

c. formula equation

d. overall ionic equation

e. net ionic equation



Sample Problem B Solution, continued

Solution: a. equation for the possible double-displacement reaction

aq + aq ? + ?3 2 4 2 4 3Zn(NO ) ( ) (NH ) S( ) ZnS( ) 2NH NO ( )→

aq + aq s + aq3 2 4 2 4 3Zn(NO ) ( ) (NH ) S( ) ZnS( ) 2NH NO ( )→


b. Table 1 reveals that zinc sulfide is not a soluble sulfide and is therefore a precipitate. Ammonium nitrate is soluble according to the table.

c. The formula equation


Sample Problem B Solution, continuedd. The overall ionic equation

aq + aq + aq + aq

s + aq + aq

2 – 2–3 4

–4 3

Zn ( ) NO ( ) NH ( ) S ( )

ZnS( ) NH ( ) NO ( )

+ +

+→

aq + aq s 2 2–Zn ( ) S ( ) ZnS( )+ →


e. The ammonium and nitrate ions appear on both sides of the

equation as spectator ions.

The net ionic equation


• Ions are formed from solute molecules by the action of the solvent in a process called ionization.

• When a molecular compound dissolves and ionizes in a polar solvent, ions are formed where none existed in the undissolved compound.

• Hydrogen chloride, HCl, is a molecular compound that ionizes in aqueous solution.

• HCl contains a highly polar bond.

aq + aq2H O –HCl H ( ) Cl ( )+⏐ ⏐ ⏐→


Ionization


Visual Concept

Chapter 13

Ionization



Visual Concept

Chapter 13

Comparing Dissociation and Ionization


Ionization, continuedThe Hydronium Ion

• Some molecular compounds ionize in an aqueous solution to release H+.

• The H+ ion attracts other molecules or ions so strongly that it does not normally exist alone.

aq + aq2H O –3HCl H O ( ) Cl ( )+⏐ ⏐ ⏐→


• The H3O+ ion is known as the hydronium ion.


Chapter 13

Hydronium Ion


Strong and Weak Electrolytes

• Electrolytes are substances that yield ions and conduct an electric current in solution.

• The strength with which substances conduct an electric current is related to their ability to form ions in solution.

• Strong and weak electrolytes differ in the degree of ionization or dissociation.


Models for Strong and Weak Electrolytes and Nonelectrolytes


Strong and Weak Electrolytes, continuedStrong Electrolytes

• A strong electrolyte is any compound whose dilute aqueous solutions conduct electricity well; this is due to the presence of all or almost all of the dissolved compound in the form of ions.

• To whatever extent they dissolve in water, they yield only ions.

• HCl, HBr, HI• All soluble ionic compounds


[HF] >> [H+] and [F–]

Strong and Weak Electrolytes, continuedWeak Electrolytes

• A weak electrolyte is any compound whose dilute aqueous solutions conduct electricity poorly; this is due to the presence of a small amount of the dissolved compound in the form of ions.

• Some molecular compounds form aqueous solutions that contain not only dissolved ions but also some dissolved molecules that are not ionized.

+⏐ ⏐→←⏐ ⏐ aq + aq–3HF H O ( ) F ( )


Colligative Properties of Solutions

• Properties that depend on the concentration of solute particles but not on their identity are called colligative properties.

• Vapor-Pressure Lowering

• Freezing-Point Depression

• Boiling-Point Elevation

• Osmotic Pressure

Chapter 13Section 2 Colligative Properties of Solutions

Vapor-Pressure Lowering

• A nonvolatile substance is one that has little tendency to become a gas under existing conditions.

• The boiling point and freezing point of a solution differ from those of the pure solvent.

• A nonvolatile solute raises the boiling point and lowers the freezing point.



Visual Concept

Chapter 13

Comparing Volatile and Nonvolatile Liquids

Section 2 Colligative Properties of Solutions


Vapor Pressures of Pure Water and a Water Solution


Freezing-Point Depression

• The freezing-point depression, ∆tf , is the difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent, and it is directly proportional to the molal concentration of the solution.

• The molal freezing-point constant (Kf ) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute.

∆tf = Kfm


Freezing-Point Depression, continuedMolal Freezing-Point and Boiling-Point Constants



Visual Concept

Chapter 13

Freezing-Point Depression


Sample Problem C

What is the freezing-point depression of water in a solution of 17.1 g of sucrose, C12H22O11, in 200. g of water? What is the actual freezing point of the solution?


Freezing-Point Depression, continued

Sample Problem C Solution

Given: solute mass and chemical formula =

17.1 g C12H22O11

solvent mass and identity = 200. g water

Unknown: a. freezing-point depression

b. freezing point of the solution

Solution:

1 mol solutemass of solute (g)

molar mass of solute (g)

amount of solute (mol)

×

=



Sample Problem C Solution, continued

Solution:


∆tf = Kfm

f.p. solution = f.p. solvent + ∆tf


Sample Problem C Solution, continuedSolution:

m

12 22 11

12 22 11

0.0500 mol C H O 1000 g water

200. g water 1 kg water

0.250 mol C H O.250

kg water

×

= =



Sample Problem C Solution, continuedSolution:

a. ∆tf = Kfm

∆tf = 0.250 m × (−1.86°C/m) = −0.465°C

a. f.p. solution = f.p. solvent + ∆tf

f.p. solution = 0.000°C + (−0.465°C) = −0.465°C



Boiling-Point Elevation

• The boiling-point elevation, ∆tb, is the difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent, and it is directly proportional to the molal concentration of the solution.

• The molal boiling-point constant (Kb) is the boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute.

∆tb = Kbm



Visual Concept

Chapter 13

Boiling-Point Elevation and the Presence of Solutes


Sample Problem E

What is the boiling-point elevation of a solution made from 20.1 g of a nonelectrolyte solute and 400.0 g of water? The molar mass of the solute is 62.0 g.


Boiling-Point Elevation, continued


Sample Problem E Solution

Given: solute mass = 20.1 g

solute molar mass = 62.0 g

solvent mass and identity = 400.0 g of water

Unknown: boiling-point elevation

Solution:

1 mol solutemass of solute (g)

molar mass of solute (g)

amount of solute (mol)

×

=



Sample Problem E Solution, continuedSolution:

1 mol solute20.1 g of solute 0.324 mol of solute

62.0 g of solute× =

m

0.324 mol of solute 1000 g water mol solute0.810 0.810

400.0 g water 1 kg water kg water× = =


∆tb = Kbm

Sample Problem E Solution, continuedSolution:


1 mol solute20.1 g of solute 0.324 mol of solute

62.0 g of solute× =

m

0.324 mol of solute 1000 g water

400.0 g water 1 kg water

mol solute0.810 0.810

kg water

×

= =


∆tb = 0.51°C/m × 0.810 m = 0.41°C

Osmotic Pressure

• A semipermeable membrane allows the passage of some particles while blocking the passage of others.

• The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration is osmosis.

• Osmotic pressure is the external pressure that must be applied to stop osmosis.



Chapter 13

Semipermeable Membrane



Chapter 13

Osmosis


Osmotic Pressure


Electrolytes and Colligative Properties

• Electrolytes depress the freezing point and elevate the boiling point of a solvent more than expected.

• Electrolytes produce more than 1 mol of solute particles for each mole of compound dissolved.

s aq + aq2H O –NaCl( ) Na ( ) Cl ( )+⏐ ⏐ ⏐→

s aq + aq2H O 2 –2CaCl ( ) Ca ( ) 2Cl ( )+⏐ ⏐ ⏐→

aq2H O12 22 11 12 22 11C H O C H O ( )⏐ ⏐ ⏐→


mol of solute particles

1

2

3

Electrolytes and Colligative Properties, continuedCalculated Values for Electrolyte Solutions

• Colligative properties depend on the total concentration of solute particles.

• The changes in colligative properties caused by electrolytes will be proportional to the total molality of all dissolved particles, not to formula units.

• For the same molal concentrations of sucrose and sodium chloride, you would expect the effect on colligative properties to be twice as large for sodium chloride as for sucrose.


Sample Problem F

What is the expected change in the freezing point of water in a solution of 62.5 g of barium nitrate, Ba(NO3)2, in 1.00 kg of water?

Electrolytes and Colligative Properties, continued


Sample Problem F SolutionGiven: solute mass and formula = 62.5 g Ba(NO3)2

solvent mass and identity = 1.00 kg water

∆tf = Kfm

Unknown: expected freezing-point depression

Solution:


mass of solute (g) 1 mol solute

mass of solvent (kg) molar mass solute (g)

molmolality of solute

kg

× =

⎛ ⎞⎜ ⎟⎝ ⎠


Sample Problem F Solution, continuedSolution:


⎛ ⎞ ⎛ ⎞×⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠

° •⎛ ⎞× = °⎜ ⎟⎝ ⎠

f

K

2

mol mol ionsmolality of solute molality conversion

kg mol

C kg H Oexpected freezing - point depression ( C)

mol ions

3 2 3 2 3 2

2 3 2 2

62.5 g Ba(NO ) mol Ba(NO ) 0.239 mol Ba(NO )

1.00 kg H O 261.35 g Ba(NO ) kg H O× =


Sample Problem F Solution, continued

Solution:


s aq + aq2H O 2 –3 2 3Ba(NO ) ( ) Ba ( ) 2NO ( )+⏐ ⏐ ⏐→


Each formula unit of barium nitrate yields three ions in solution.

Electrolytes and Colligative Properties, continuedActual Values for Electrolyte Solutions

• The actual values of the colligative properties for all strong electrolytes are almost what would be expected based on the number of particles they produce in solution.


Electrolytes and Colligative Properties, continuedActual Values for Electrolyte Solutions

• The differences between the expected and calculated values are caused by the attractive forces that exist between dissociated ions in aqueous solution.

• According to Debye and Hückel a cluster of hydrated ions can act as a single unit rather than as individual ions, causing the effective total concentration to be less than expected.

• Ions of higher charge have lower effective concentrations than ions with smaller charge.


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Dissociation Dissociation is separation of ions that occurs when an ionic compound dissolves. Chapter 13 Section 1 Compounds in Aqueous Solution 1 mol